There are generally two reasons why this happens. For one, ProTools gives this message any time our authorization process doesn't complete. Please click "I'm online" and enter your login info when the Activation prompt comes up during startup. If you click "Quit" instead, you'll get that "not a valid 64-bit plugin" message.
It's also possible that the plugin somehow did not install properly, and the AAX file became corrupted. If that's the case, you can manually uninstall it and then reinstall it. Just go to this directory location:
C:\Program Files\Common Files\Avid\Audio\Plug-Ins
This will open a folder with all your AAX plugins, and you can trash the plugin file. Then try installing it again.